The present invention relates generally to the formation of electrical conductors in holes of relatively narrow diameter and extended length. More specifically, it relates to formation of electrical conductors in holes through insulating materials which had diameters of the order of 1 micron and which have lengths which are a factor of 6 or more times the diameter of the hole.
It is known that laser drilling can produce holes with length-to-diameter ratios which are relatively high for small diameter holes. Satisfactory methods exist for forming electrical conductors or feed-throughs in holes of insulating substrates for holes whose length-to-diameter ratio is less than 6. In other words, where the diameter of the hole is of the order of a micron in size and the length of the hole is about 6 microns corresponding to the thickness of the material in which the hole is formed, the conventional practices now in use are successful in forming a conductive bridge through the hole from one wafer surface of the material to the opposite wafer surface. The methods which have been employed in successfully bridging the two surfaces of an article include capillary wetting, wedge extrusion, wire insertion, electroless plating, electroforming and double-sided sputtering in conjunction with through-hole plating. These are among the various methods which have been tried to implant a conducting electrical interconnection in a micron diameter hole whose length is much greater than its diameter. However, when the length is more than 6 times greater than the diameter, these methods are not successful in forming a conductor or implanting a conductive path through the material from one major surface to an opposite parallel major surface on the other side of a disk, for example. For micron-diameter holes with a length-to-diameter ratio greater than 6, none of the known methods has been found successful for implanting a conductor with a high yield.
Electrical interconnections through such holes or in such holes are important because laser drilling can produce holes with length-to-diameter ratios which are quite high and as high as 50. The smallest possible hole diameter is desired in many structures such as electronic circuitry in order to conserve available surface area of the structures, also sometimes referred to as conserving real estate. Surprisingly, I have now found a simple inexpensive method of forming an electrically conductive coating on the walls of micron-sized holes with very large length-to-diameter ratios and in similar small diameter holes with large length-to-diameter ratios.